A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The demand for ever-smaller features to be imaged with lithographic apparatus has resulted in the use of projection systems with increasing numerical aperture (NA). The angle of rays of radiation within the projection apparatus with respect to the optical axis increases with increasing NA. The vector nature of light becomes important for imaging because only identically polarized components of electromagnetic waves interfere. Therefore it is not the wavefront quality alone that determines the image contrast; the polarization has a considerable influence as well. Furthermore, the use of illumination radiation having specifically desired states of polarization for specific regions is increasingly being used for imaging features aligned in particular directions. Consequently, it is desirable to know the state of polarization of the radiation impinging on the patterning device, such as a reticle. It can also be desirable to know the effect on the state of polarization caused by the projection lens. However, no suitable measurement system currently exists. Existing radiation sensors built into lithographic apparatus are generally polarization insensitive. Furthermore it is thought that the state of polarization of the illumination radiation at the level of the patterning device cannot be measured at the level of the substrate without knowing the effect of the projection lens on the polarization. One way to avoid this would be to insert a polarization measurement system at the level of the reticle before the radiation enters the projection lens. However, this has the considerable problem of designing a measurement system to fit into the limited space and also to provide power and data connections to the measurement tool, either electrically or wirelessly, which would require significant down-time for installation, which is expensive. Reflection and transmission at interfaces and coatings and birefringence of the lens element materials are possible sources for polarization changes in the projection system. However, as previously explained, no measurement technique exists to characterize the polarization effects of the projection system.